Magnetic tunnel junctions (MTJs) are one of the basic building block of spintronics; they are made up of at least a thin oxide barrier (typically magnesium oxide or alumina) which separates two ferromagnetic layers. One of these ferromagnetic layers is called the reference layer (RL) and has its magnetization fixed in space thanks to a high magnetic anisotropy or a strong exchange coupling to another layer. The second ferromagnetic layer is called the free layer (FL) and its magnetization can be rotated in space between two stable states.
The resistance of a MTJ depends on the relative orientation of the FL and RL due to the tunneling magnetoresistance (TMR) effect. In recent years, it has been found that magnetic momentum can be transferred from the RL to the FL and vice versa, depending on the polarity of the electrical current applied to the MTJ, across a thin oxide layer; where this effect is called Spin Transfer Torque (STT). An MTJ therefore forms the basic structure of an STT-MRAM (Magnetic Random Access Memory), in which the data bit is written by STT and read by TMR. In typical STT-MRAM devices, a bipolar electric current of the order of 1-10 MA/cm2 is applied across an MTJ during 5-10 ns to write a data bit, which results in an energy consumption of the order of 0.1-0.3 pJ.
One of the big challenges the STT-MRAM is facing is the high writing current density (Jc), while maintaining high thermal stability (A) as the size of the memory cell shrinks. Recent studies have shown that an electric-field (E-field) can change the perpendicular magnetic anisotropy (PMA) of a magnetic layer in MTJs and such an E-field effect is proven to be the alternative energy-efficient (corresponding to a Jc down to 104 A/cm−2) route to manipulate the magnetization of a magnetic layer as compared to the usual magnetic field and STT switching.
So far, only two writing schemes employing the E-field effect have been described in the literature: (1) Bistable magnetic switching using a combination of unipolar voltage-induced coercivity (Hc) reduction, STT and an intrinsic or external magnetic bias field, and (2) Precessional switching by a combination of an intrinsic or external magnetic bias field and a unipolar voltage pulse-induced dynamic magnetization switching. While the former is limited by the operation speed due to the thermally activated switching process and the switching reliability, the latter is very sensitive to the pulse duration and requires an in-plane bias magnetic field. These two schemes dramatically suffer from a large write error rate (WER) because of the narrow window of the E-field dependency on the voltage, pulse width and bias field, and it is thus nearly impossible to develop a process in order to implement such writing schemes in practical stack structures.
Some of the challenges in the prior art include the requirement of precise control of the pulse duration, switching pulse duration which may be sensitive to the cell dimensions and the need for an external in-plane field as the intrinsic anisotropy field leads to uncontrolled switching.
Therefore, there is need to provide magnetic switching having a large margin for pulse duration, and/or robust against process, and/or where no external field is required.